Re^2: Need some wisdom on strings to numbers

swampyankee asked whether I would consider this a bug to be filed against Data::Dumper. As I have no decided opinion on the matter, I'd like to pass the question on to all of you... Essentially, I see two issues:

(1) Data::Dumper promises

(...) The return value can be "eval"ed to get back an identical copy of the original reference structure.

However:

use Data::Dumper;
use Devel::Peek;

my $foo = { num => 123.1 };
Dump $foo->{num};

my $serialised = Dumper $foo;
$foo = eval $serialised;   # deserialise
Dump $foo->{num};

__END__
SV = NV(0x669d40) at 0x63c430
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = 123.1               <-- originally, it's a number

SV = PV(0x63d918) at 0x742c80
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x6650d0 "123.1"\0  <- it's a string now, when deserialised...
  CUR = 5
  LEN = 8
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But what would you consider "identical"...? I.e. does internal representation really matter, as Perl would automagically convert under most circumstances, anyway?

(2) It's not exactly good manners to silently modify data structures under the hood, when being passed them just for dumping purposes (this could cause unexpected issues with copy-on-write semantics with forked processes — such as when trying to preload libs/data to be shared across processes, in order to keep memory usage low (mod_perl comes to mind...)). OTOH, this is not the only case in Perl where such things do happen...

So, whuttaya think?

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Re^3: Need some wisdom on strings to numbers by swampyankee (Parson) on Oct 09, 2009 at 21:17 UTC
Personally, I would consider silently modifying data structures to be a bug. I realize there may be reasons for Data::Dumper treating floats as strings, mostly because Perl relies on the math library used by the C compiler used to build Perl's executable, and it may be distracting to see 123.1 show up as 123.09999… in Data::Dumper's output. The most obvious fix is to modify the documentation ("Data::Dumper outputs all floating point values as strings so that 123.1 doesn't appear as 123.0999…"). This has the obvious advantage of avoiding the need to change any code. Information about American English usage here and here. Floating point issues? Please read this before posting. — emc	[reply]

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Re^3: Need some wisdom on strings to numbers
by swampyankee (Parson) on Oct 09, 2009 at 21:17 UTC

Personally, I would consider silently modifying data structures to be a bug. I realize there may be reasons for Data::Dumper treating floats as strings, mostly because Perl relies on the math library used by the C compiler used to build Perl's executable, and it may be distracting to see 123.1 show up as 123.09999… in Data::Dumper's output. The most obvious fix is to modify the documentation ("Data::Dumper outputs all floating point values as strings so that 123.1 doesn't appear as 123.0999…"). This has the obvious advantage of avoiding the need to change any code.

Information about American English usage here and here. Floating point issues? Please read this before posting. — emc

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